This invention relates to process time controllers and more particularly to a rate-type process time control system of digital design for automatically controlling the process time for a product of traverse a given process length.
There are numerous applications for process time controllers where it is important to determine and control with a high degree of precision the time during which some kind of product, such as food or any of various objects, bodies, substances or matter must undergo processing, such as heating, cooking, treating, painting, machining, washing, scrubbing, polishing, or the like, by moving such product along a process length.
Where the product is carried along the process length by means of an electric motor-driven conveyor, it has been known to use a variable speed electric motor such that the speed of the motor is initially established at a value which it is hoped and intended will cause the product to traverse the process length in a given amount of time.
It has also been known to use feedback-type motor speed controls to provide improved regulation of the motor speed. When such simple feedback systems are used for process time controller applications, good process timing may result only if motor speed is tightly regulated and only if the proper speed can be first determined. However, great difficulty often arises in establishing the initial motor speed which must be used to provide a desired process time for a given process length. Even if a motor speed can be determined for one desired process time, it may be most difficult to determine a motor speed if a different process time is desired.
Moreover, conventional motor speed control systems may suffer certain other problems when employed in process time control applications. For example, accuracy or response of the speed control system may vary with motor speed, i.e., may be nonlinear in nature; the speed regulating characteristics of feedback systems may vary with changes over time in the values of component or may vary with changes of temperature; special motors or expensive or cumbersome motor control components may be required; speed regulating characteristics of motor control systems may be useful only over a limited speed control range; there may be objectionable accelerations in changing speeds or at start up or termination of the process; or the motor torque may be insufficient at low speeds. In addition, prior art systems may permit process time errors by falling to provide adequate compensation of process time with fluctuations of power supply voltage.
The present invention is disclosed in the form of a system particularly useful for controlling the time during which food is processed in a commercial food baking oven, e.g., of the multiple tier type disclosed in U.S. Pat. No. 4,008,996 of Harold D. Wells, entitled "Multiple Tier Oven", although the invention may be used in many other types of situations requiring control of process time with a high degree of precision. In this kind of application, as in many others, it is highly desirable to permit the process time to be set directly by the user, without resort to the operator having to first make conversions to ratios or motor speeds, or to estimate or arbitrarily select or relative values, and without requiring guessing or estimating on the part of the operator.